The invention relates to a method for the laser remote welding of at least two coated sheets, wherein the laser beam is directed onto the sheets to be joined to one another and guided thereon by way of a scanner device. Further, the invention also relates to a system for carrying out the method.
A method of the kind mentioned in the introduction is disclosed in DE 103 09 157 B4. With the previously disclosed method, the coated sheets to be joined are positioned on top of one another, as far as possible without a gap. During a first method step, the sheet facing the laser beam is first heated by the laser beam, wherein the coating of both sheets vaporizes on their mutually facing sides and is distributed between the sheets without the basic material of the sheets melting. During a second method step, the two sheets are welded by the laser beam in the region where the coating has been removed and in this way joined to one another by material-to-material bonding.
In addition, reference is also made to JP 11047967 A with regard to the prior art.
The invention is based on the object of providing a method of the above-mentioned type that does not have, or at least minimizes, the disadvantages which accompany the prior art.
This and other objects are achieved by a method, and corresponding system for carrying out the method, for the laser remote welding of two coated sheets, wherein a laser beam is directed onto the sheets to be joined to one another and guided on the sheets by way of a scanner device. With the method according to the invention, the coated sheets are joined to one another by producing an end fillet weld on a lap joint, wherein the continuous production of the weld is recorded by (at least one) imaging unit and, if necessary, the path of the laser beam (on the sheets to be joined) is corrected and matched to a desired weld course on the basis of an automatic evaluation of recorded images (or a recorded video sequence or similar). In particular, this takes place during the ongoing welding process.
In particular, “laser remote welding” is understood to mean a welding process using scanner technology or a scanner. Here, the laser beam is deflected by at least one movable rotating mirror or scanner mirror, and is positioned and guided onto the sheets to be joined, whereby very high processing speeds can be realized. The scanner can also have lens systems for focusing the laser beam. The welding operation is typically carried out without filler metals.
An “end fillet weld on a lap joint” is particularly understood to mean a welded joint between at least two sheets, wherein a top sheet rests on a bottom sheet in an at least partially overlapping manner and the weld is formed between at least one edge surface or side surface of the top sheet and the adjacent contact surface of the bottom sheet. Such a weld can also be referred to as a “side fillet weld”. The end fillet weld can be formed as a continuous weld or in the manner of a stitch weld (i.e. with discontinuities in the weld).
The sheets to be joined to one another can be flat sheets (plates) or spatially formed sheets (shaped sheet metal parts). Preferably, it is provided that the sheets are zinc-coated steel sheets. Further, the sheets can also be aluminum-coated steel sheets. A zinc coating or aluminum coating is particularly understood to mean a zinc or aluminum-based anti-corrosion coating.
The provision according to the invention of an end fillet weld leads to very good degassing conditions for the coating material (in particular zinc), as a result of which disturbances to the process are significantly reduced compared with the prior art mentioned in the background section. In addition, with the method according to the invention, only one operation is required; the heating by the laser beam (for vaporizing the coating) described in DE 103 09 157 B4 is superfluous.
However, critical deviations from a specified desired weld course can occur when producing the end fillet weld, for example as a consequence of component tolerances. In order to prevent such deviations, according to the invention, an optical monitoring of the weld course (weld position detection by recording and evaluating images) and a corrective movement for the laser beam possibly resulting therefrom are also proposed, wherein the corrective movement is realized in particular by the scanner or by the scanner mirror fitted therein, so that the laser beam can precisely follow the specified desired weld course. This can also be referred to as online regulation of the weld course (or as online weld tracking). The method according to the invention therefore enables robust laser remote welding of at least two coated sheets, which is also suitable particularly for serial production in vehicle body construction.
A further advantage of the method according to the invention is to be seen in that the two coated sheets to be joined are welded without a gap or without spacing (i.e. with so-called zero gap) by means of the at least one fillet weld. A gap between the welded sheets is undesirable, as dirt and/or other corrosive media (in particular moisture) can penetrate, this being promoted by a capillary effect. With the method disclosed in DE 103 09 157 B4, the coating vapor condenses between the sheets, which necessarily leads to a spacing between these sheets. With the method described in JP 11047967 A, a spacing between the sheets (by means of melt craters) is provided from the outset, wherein the introduction of the melt craters for adjusting the gap constitutes a separate and elaborate operation.
The images or video sequences produced by the at least one imaging unit, such as a camera, and subsequently evaluated by way of a control unit are, in particular, grayscale image recordings. Preferably, it is provided that the region recorded by the camera is additionally illuminated by at least one artificial light source, which, for example, is of advantage with regard to smoke and vapor or a larger camera distance. The artificial light source can be built into the scanner, for example, or fixed externally on the scanner. Likewise, the camera can also be built into the scanner or fixed externally on the scanner.
The recorded images can be evaluated with regard to the weld course with reference to the step offset between the sheets, which is necessarily present on account of the overlap. As a result of the step offset, the position of the generated fillet weld can be easily identified on the recorded images and correlated with the step offset. This then allows a determination of the actual position of the laser beam and the calculation of a corrective movement, which is then realized by appropriate conversion by use of the scanner or the scanner mirror incorporated therein.
As an option, in a first method step, it can be provided that initially only the coatings (of the sheets) are vaporized with the laser beam (i.e. removed by vaporization) in the region of the end fillet weld to be applied, and that then, in a second method step, the sheets are welded with the same laser beam in the areas with the coating removed. The different energy contribution required in the individual method steps can, for example, be adjusted by the traversing speed of the laser beam (and/or, if necessary, also the number of traversals). Further, it is preferably provided that, in the first method step, the laser beam is actually widened by defocusing or oscillation, so that the coating can be vaporized in a relatively wide strip (without the sheet base material otherwise melting).
Alternatively, it can be provided that the coatings (of the sheets) are vaporized with a separate (second) laser beam in the region of the end fillet weld to be applied, wherein this separate laser beam leads the (first) laser beam for the welding operation. Accordingly, two laser beams are required here, but only one operation. Two laser beams can be implemented, for example, by two scanner devices or by so-called double-focus technology.
The images or video sequences recorded by the camera can also be automatically evaluated with regard to checking the quality of the generated end fillet weld, for example with regard to spatter, pores, cracks and the like. The automated evaluation can be undertaken by the control unit. This enables the quality of the generated end fillet weld to be checked by automated inspection without significant increase in effort or cost. Various possibilities for visual inspection of the weld quality are known from the prior art.
Preferably, it is provided that prior to producing the end fillet weld or a plurality of end fillet welds, first the desired weld course is determined or defined, which, among other things, can include a shortening and smoothing of the weld course, an optimization of the speed, an accessibility and collision check, a cycle time optimization and/or a sequence planning of a plurality of end fillet welds. Preferably, this is CAD-based. This can also be referred to as off-line path planning.
The system according to the invention for the laser remote welding of two coated sheets comprises:                a scanner, by which the (at least) one laser beam can be directed and guided onto the sheets to be connected or joined to one another;        (at least) one imaging unit, with which the continuous production of the weld (of an end fillet weld) by way of the laser beam can be (optically) recorded; and        a control unit, such as in particular an appropriately programmed computer, with which the images recorded by the imaging unit are automatically evaluated and, based thereon, if necessary, the path of the laser beam is corrected and matched to a desired weld course.        
Preferably, it is provided that the scanner is mounted on a movable mechanism. A movable mechanism is, for example, a robot, a gantry or the like. The working area can be increased by moving the scanner to different positions by way of the movable mechanism. If the traverse movements of the scanner take place simultaneously with the welding operation, this is referred to as welding “on the fly”, as a result of which processing times can be considerably shortened. On the fly welding requires a synchronization of the movements (i.e. movement of the laser beam by the scanner and movement of the scanner by the mechanism), which can be taken into account both with the off-line path planning and with the online weld tracking. The control device is then designed or constructed appropriately for this purpose.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.